The present invention relates to a deposition apparatus or vacuum-deposition apparatus for forming layers of an organic light-emitting element used as a display device.
The electro-luminescent element exhibits high visibility due to characteristics of self-luminosity. The electro-luminescent element exhibits excellent resistance against the mechanical impact, since the electro-luminescent element is made of a complete solid-state material. The electro-luminescent element is used to various display devices by virtue of these merits. The organic thin-film light-emitting element, which is an organic electro-luminescent element, has been attracted much attention, since the organic thin-film light-emitting element facilitates greatly lowering the necessary driving voltage and all the colors may be illuminated by appropriately combining various light-emitting materials.
Especially, after Tang et al. disclosed a laminate-type thin-film light-emitting element which emits light with high-luminance of 1000 cd/m.sup.2 at a low applied voltage of 10 V (cf. Applied Phys. Lett., 51,913 (1987)), the developments of practical element structures, various materials and manufacturing techniques have been conducted actively.
FIG. 1 is a cross section showing a layer structure in a typical organic thin-film light-emitting element. Referring now to FIG. 1, the thin-film light-emitting element includes an inorganic anode layer 2 deposited under an optically transparent substrate 1, an organic or inorganic hole injection layer 3 under the anode layer 2, an organic light emitting layer 4 under the hole injection layer 3, an organic or inorganic electron injection layer 5 under the light emitting layer 4, and an inorganic cathode layer 6 under the electron injection layer 5. The thin-film light-emitting element is used in a display device by applying a drive voltage supplied from an external power supply 7 between the anode 2 and the cathode 6 such that light generated from the light emitting layer 4 may be controlled.
It is required for the organic light emitting material to be easily formed in a form of a film, to exhibit a high light-emitting efficiency, and to be stable. It is required for the charge injection material to be easily formed in a form of a film, to be highly efficient in charge transport and charge injection into the light emitting layer, and to be stable. Japanese Unexamined Laid Open Patent Applications (KOKAI) No. H02-311591 and No. S59-194393 disclose the preferable materials.
Although vapor phase growth method and liquid phase growth method are applicable to the deposition of the inorganic layer and the organic layer, the vapor phase growth is used generally in depositing the organic and inorganic layers (hereinafter referred to as "constituent layers") of the organic thin-film light-emitting element. Since the organic materials for forming the organic layers are soluble generally into organic and inorganic solvents, it is difficult to deposit the organic layers by the liquid phase growth method.
FIG. 7 is a cross section of a conventional batch-type vacuum-deposition apparatus for depositing a thin layer. FIG. 8 shows cross sections of a conventional single-wafer-processing-type vacuum-deposition apparatus for depositing a thin layer. FIG. 9 is a cross section of a conventional transfer-type vacuum-deposition apparatus for depositing a thin layer.
The apparatuses of FIGS. 7, 8 and 9 include three sources for depositing a thin film consisting of three layers. In the batch-type vacuum-deposition apparatus of FIG. 7, the constituent layers of an organic light-emitting element are deposited successively in respective film-deposition chambers 10a, 10b and 10c. The chambers 10a, 10b and 10c are connected via vacuum valves 11. Sources 8a, 8b and 8c are located below respective shutters 12 in the respective chambers 10a, 10b and 10c. In the single-wafer-processing-type vacuum-deposition apparatus of FIG. 8, sources 8a, 8b and 8c are disposed in a chamber 10, from which a substrate 9 is taken out after a three-layered film is formed thereon. Although the sources 8a, 8b and 8c are located below respective shutters 12 in a chamber 10 in the transfer-type apparatus of FIG. 9, in the similar manner as in the apparatus of FIG. 8, a plurality of thin layers is deposited continuously in the transfer-type apparatus, since the substrate 9 is located on conveyer means in the apparatus of FIG. 9.
It is difficult for the vacuum deposition apparatuses of FIGS. 7, 8 and 9 to deposit the constituent layers of an organic light-emitting element with high rate of operation due to the following three reasons.
(1) In depositing organic or inorganic layers in a vacuum deposition apparatus, it is necessary to conduct a maintenance operation for cleaning the parts equipped in each chamber and the inside wall of the chambers except the substrate after every deposition on the substrate.
(2) It is necessary to initialize (degas) the sources after a new source of an organic or inorganic material (hereinafter referred to as "constituent material") has been loaded or after the source or sources have been exposed to air by breaking the vacuum condition of the vacuum chamber.
(3) The film deposition should be interrupted when it becomes impossible to keep using the source, e.g. due to exhaustion of the source material.
The foregoing reasons (1) and (2) will be explained below in more detail.
At first, the reason (1) will be explained.
In the step of depositing the constituent material on the substrate in the vacuum deposition apparatus, the constituent material evaporating from its source accumulates in the form such as flakes on the following five constituent parts of the vacuum deposition apparatus except the substrate.
The part 1: a source including a boat or crucible for heating an organic or inorganic material and a constituent material to be evaporated by resistance heating in a vacuum condition.
The part 2: a mask for controlling or limiting the vapor flow of the constituent material into a predetermined pattern to deposit the constituent material in accordance with the predetermined pattern on the substrate.
The part 3: a shutter for allowing the vapor flow of the constituent material to pass or block, used at the start or end of depositing the constituent material on the substrate.
The part 4: a sensor for monitoring the thickness of the deposited constituent material layer and the deposition rate of the layer.
The part 5: a shielding cover for preventing the constituent material from accumulating on the inside wall of the vacuum chamber.
The organic material accumulated on the shielding cover, the mask, the shutter, the sensor and the source tends to peel off inside the apparatus due to the thermal instability, the low density, the insulative nature thereof and the large differences in the coefficient of thermal expansion between the organic material and the inorganic materials such as aluminum and stainless steel, which is used mostly for the above-described parts.
If the parts of the vacuum deposition apparatus once accumulated with the organic material are used in the next step, the organic material which has peeled off the parts will be contained in the organic light-emitting element as the powdery dust.
The size of the powdery dust is in the range between several .ANG. and several mm depending on the manner of the peeling, the material thereof, the vaporization rate of the organic material and the structure of the vacuum chamber. On the other hand, the constituent layer of the organic light-emitting element is very thin and the thickness thereof is in the range between several tens nm and several hundreds nm. Therefore, even if the powdery dust in the layer thickness size is included in the light-emitting element, it causes dark spots due to the coagulation of the organic materials, a leakage current due to localization of the electric field, and chemical change of the organic layer, so that shortening of life of the light-emitting layer, increasing of poor portions in the light-emitting, luminance fluctuation and color fluctuation are caused. Therefore, the inclusion of the powdery dust in the light-emitting element causes serious problems on the development of the organic thin-film light-emitting element.
It is difficult to improve the rate of operation of the vacuum deposition apparatus for forming the thin layer and the costs of manufacturing of the organic thin-film light-emitting element soar, since it is necessary to frequently clean the parts equipped in the vacuum chamber on which the organic material is accumulated.
Now the reason (2) will be explained below.
It is necessary to break the vacuum condition in the vacuum chamber for the cleaning of the parts in the steps of depositing the constituent layers. Once the vacuum condition is broken, gas components, such as hydrogen, water, carbon monoxide and carbon dioxide, are absorbed into the sources. In forming the thin layer, the vacuum chamber is heated up to a certain soaking temperature, kept at the soaking temperature and cooled to degas the source before the deposition. If this degassing operation (maintenance operation) is omitted, the gas constituents, such as hydrogen, water, carbon monoxide and carbon dioxide, are contained in the organic light-emitting element.
The inclusion of the gas constituents causes lowering of the injection efficiencies of the electron and the hole injected from the respective electrodes to the light-emitting layer, and chemical change of the light-emitting layer. Since the lowering of the injection efficiencies and the chemical change further cause lowering of the luminance of the organic thin-film light-emitting element, the inclusion of the gas constituents poses serious problems impeding the practical use of the organic thin-film light-emitting elements.
When a new source is loaded, a uniform constituent layer will not be deposited immediately after heating the new source, if a specific degassing operation (initializing operation) is not conducted.
Thus, it is difficult to improve the rate of operation of the vacuum deposition apparatus for forming the thin layers and to lower the manufacturing cost of the organic thin-film light-emitting element, since it is necessary to conduct the foregoing maintenance operation frequently and since it is necessary to conduct the above described initializing operation including degassing every time when the source is replaced to new one.
One batch process in the conventional vacuum deposition apparatus needs all of the steps of cleaning the constituent parts contaminated with dust, loading the cleaned constituent parts into the vacuum chamber, evacuating the vacuum chamber, initializing the sources, heating the sources, depositing a constituent layer on a substrate, changing the substrates and removing the constituent parts contaminated with dust, as described in FIG. 10.
In view of the foregoing, it is an object of the invention to provide a deposition apparatus for an organic thin-film light-emitting element which obviates the foregoing problems of the conventional apparatuses.
It is another object of the invention to provide a deposition apparatus which facilitates manufacturing of the organic light-emitting element which contains less defects, at a high rate of operation thereof.